Piezoelectric based measurement devices typically have a thickness-shear resonator located in a housing with electrodes. The thickness-shear resonator is generally made from quartz owing to its excellent stable properties under temperature and pressure, low hysteresis, high resolution, high accuracy, and good long-term stability. A thickness-shear resonator is useful in that it can be excited into resonance through the application of an external electric field, which is preferably applied to the resonator through electrodes formed thereon by means of vacuum deposition of conductive metals, such as copper, silver, or gold. The resonator frequency (or frequencies if the resonator is excited in both the thickness-shear modes of vibration) is dependent on the elastic coefficients, density, thickness, and overtone operation of the resonator. The resonator frequency shift in relation to changes in temperature, pressure, or externally applied force transmitted to the resonator via the housing. The capability of a thickness-shear resonator to experience a shift in its resonance frequency is quite useful in sensor applications (but not in frequency control) if the resonator is responding essentially to only one of the variables of temperature, pressure, voltage, or externally applied force within a relatively narrow operating range. If the resonator is responsive to more than one variable in its intended operating range, then the variable(s) other than the one being measured is compensated in order to successfully measure the desired variable. On the contrary, for frequency control applications, the thickness-shear resonator should not experience a shift in its resonance frequency in relation to changes in temperature, pressure or externally applied forces (for example, the stress caused by the mounting supports at their boundaries with the resonator).
Both theoretical and experimental results show that the frequency changes induced in a thickness-shear quartz resonator due to changes in temperature, pressure or externally applied forces are strongly dependent on its crystalline orientation, resonator geometry, material anisotropy, mounting supports, and aspect ratio. Recent discovery of Stress Compensated (SC-cut) and Stress Compensated B-mode Temperature Compensated C-mode (SBTC-cut) orientations of quartz have helped to significantly minimize these effects. The discovery of the SBTC-cut for the thickness-shear quartz resonator provided for the development of a dual-mode concept, which, in turn, led to the development of precision dual-mode quartz pressure sensors. Dual-mode based sensors utilize the fast thickness-shear mode (B-mode), which is stress compensated, and the slow thickness-shear mode (C-mode), which is temperature compensated. The frequency of the B-mode indicates primarily the temperature of the sensing resonator, and the frequency of the C-mode indicates the applied pressure. As the B-mode's frequency depends on the temperature of the resonator's vibrating volume, the effects of temperature gradients are greatly reduced. Thus, under pressure and temperature transients, dual-mode sensors allow for superior temperature compensation accuracy and superior pressure sensing accuracy. The discovery of SC-cut for the thickness-shear quartz resonator let to the development of precision frequency control devices and applications requiring frequency stability, as it exhibited superior frequency-temperature stability over narrow temperature ranges to obtain good static compensation and frequency-stress stability.
Thus, the discovery of stress and temperature compensated crystalline orientations of quartz facilitated the development of a sensor with superior sensing characteristics and stable properties at its resonant frequency, even when subjected to thermal and mechanical stresses. This technology has been deployed for high precision pressure and temperature sensors in the oilfield services industry for the last two decades. The sensors provide reliable estimates of formation properties, such as pressure and temperature. These basic properties are used to determine other formation properties, like permeability and oil/water interfaces in the formation, which, in turn, are used to facilitate optimal completion of wells for oil and gas production. In addition, the quartz resonators are used as frequency control devices in the oilfield for timing (telemetry) applications.
Although quartz has been a valuable resonator material in the oilfield services industry, the low phase transition temperature of quartz, which occurs at 573° C., limits its application up to 250° C. Thus, quartz resonators cannot be used for certain oil and gas field applications. For example, quartz resonators cannot be used for deeper and more productive oil/gas exploration because of the high temperatures associated with drilling deeper into the Earth.